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Talk about Tq

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Talk about Tq

Old 15th Nov 2008, 01:14
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Talk about Tq

Hi,

I have a wager with someone. Perhaps you can tip the bet in my favour.

On my helicopter the tq is measured at the engine. The engine(s) can produce more Tq than the transmission can handle. In this experiment we will continue to raise the collective passed the transmission limits and see what happens to the Tq display once the limits are reached. There are limits of TgT, Nf and Tq after which the FADEC fuel limits and the rotors droops.

Simple question, after a limit is reached, I raise the collective further and the rotors droop, what happens to the Tq? Does it rise as my simulator displays, or should it stay reasonably static because the engine is limiting? Is there a difference on which limit is kicking in?

Perhaps you need more info to reach your conclusion, ask and I will supply the details.

I raise the collective and Tq continues to rise but the Nr decays, or I raise the collective and Nr reduces and Tq stays static at the limit. Which one do you think will happen, and why?

Jeep
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Old 15th Nov 2008, 07:12
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Jeep

I assume you are talking about a twin spool engine and mean TGT, Ng and Tq. Not Nf - the power turbine speed.

If it is a FADEC controlled engine TGT and Ng will (should) stay constant once the respective limit has been reached and the fuel flow will become constant (i.e. power limited). In the case of a true tq limiter, the tq will stay constant and the fuel flow will gradually reduce as the rotor droops away from the set datum.

Some engines (OK- I know of one) will have a knee point in their tq-droop schedule (about 15-20% below rotor datum) where the tq limit reverts to constant power i.e. the tq starts to rise again with further rotor droop. This is supposedly to maintain sufficient power to recover (accelerate) the rotor (at a very high blade pitch angle) back to datum from such a low position although it would appear to be a bit academic at that low level as the NR at that point is typical of the minimum NR seen following a successful vertical reject from the limiting Cat A hover height. But then again if you really, really need to keep on flying then sod the Tx limits....

lhb
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Old 15th Nov 2008, 08:27
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Think I know what your after jeep,
Torque is a measurement of how much something is being twisted, the physics says it is proportional to amount of force and moment arm from axis of rotation. That is for some thing static. If it is spinning the the power required to produce that torque is proportional to the torque required and the rotational speed.
In a helicopter if you are using torque measured at the engine or its input to the gearbox at the point when the engine is at max everything then as you increase collective, overpitching the rotor blades, as the Nr decays the torque measured at the negine will increase despite no change in engine speed.
We used to practice single engines failures in the SeaKing by setting one engine in manual (fuel computer off line and Ng/TIT fixed) then backing off the computer controlled engine to practice single engine flyaway or landings.
It is quite obvious that as Nr decays torque is increasing despite the driving engine at constant speed. The set up actually allowed for this to avoid transient single engine overtorque.
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Old 15th Nov 2008, 11:25
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There we have both sides of the argument. But which one is correct for the RTM on the Apache?
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Old 15th Nov 2008, 12:22
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Jeep

Check your PM's.

lhb
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Old 15th Nov 2008, 12:30
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Can't answer about an apache but our 225s have FADEC stops determined by the first limiting parameter, either torque or N1. In twin engine mode in UK at low level its always torque. Max takeoff torque is 100% with a 20 sec transient to 110% - at which point the FADEC stop is reached.

We had an incident where someone had to pull the collective past the 110% point, at which point the Nr of course started to decay. I think he went from 103% to about 96%. The torque remained at 110% - the engine power was reduced by the FADEC to compensate for the increasing torque resultant from the lowering N2.

So no damage was done, no limits exceeded.

On the 225 its only when you get down to 90.5% that the FADEC decides to give you more power - it arms OEI 2 minute rating on both engines giving you an increase in torque that will damage the transmission but hopefully stop you from crashing.

HC
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Old 15th Nov 2008, 20:26
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Perhaps it might help if the relation between power, fuel flow and torque is understood.
Power is torque multiplied by rotor RPM times an arbitrary constant (the constant differs from helicopter to helicopter).
And fuel flow is directly related to power.
The other thing to consider is that torque is really the measure of drag on the rotor blades.
So if power / fuel flow is constant due to a fuel control preventing the fuel flow from increasing, and the collective is raised to increase the pitch on the rotor blades (and hence increase the torque), the rotor RPM will decrease.


The Bell 212 has a torque limiter that will reduce fuel flow whenever the torque is above the value set by the limiter. You can decide for yourself if this is a good idea or not. I think the UK versions had some minor change to the fuel control to make the torque limiter do something different.

Hope that helps to obfuscate the confusion.
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Old 15th Nov 2008, 22:14
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From memory I think the RTM Apache Tq limiter is a genuine Tq limiter so as Nr decreases Tq will remain approximately constant until it eventually releases (probably). With a Sea King engine in manual the engine is power limited (not Tq) so as Nr decreases Tq will rise to maintain constant power.
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Old 17th Nov 2008, 19:58
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Very interesting answers.

Shawn I had to look up that word

LBH i have replied to your PM.

I think it is becoming clearer to me. The TQ limit in this case is set at approx 150%. If that limit is reached, the collective can be raised and Nr will decay but the TQ should not rise (display) because that is the limit.

If a TGT or NG limit is reached before the TQ limits, then raising the collective will still incur an Nr droop, but TQ will continue to rise.

Of course in both cases there is a TQ rise but the display to the crew will depend on the output from the FADEC which goes to a gauge (display). TQ limit stops TQ rising?

So I wonder what is the difference in the two scenarios? How does the FADEC limit the engine output to the transmission in both of those cases? How can the FADEC stop the increase in TQ passed the TQ limit without an increase in 'total' TQ ie TQ limit and overpitched NR droop?
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Old 17th Nov 2008, 20:12
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The torque you see is not a FADEC output, it is an actual torque measured on the high speed shaft. When the FADEC limits on Ng or TGT you have reached a power limit so the fuel flow is effectively frozen and if you demand more power, torque will rise and Nr will droop. When you reach the torque limit (which from memory is a genuine Tq limit but I could be wrong) the fuel flow is varied to maintain constant torque. As you demand more power Nr droops, fuel flow decreases and torque remains constant. If I am wrong and the ultimate 'torque limit' is actually a power limit then torque will rise as Nr droops just as it does when you reach an Ng or TGT limit.
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Old 17th Nov 2008, 21:03
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adrs,

thanks for that, most helpful. The picture becomes clearer.
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